Photoconductive control circuit



Nov. 17, 1959 c. D. BAYLEY 2,913,637

PHOTOCONDUCTIVE CONTROL CIRCUIT FiledDeo. 28, 1955 IO M LOAD 5 l 13 I 1m m PHOTO-CONDUCTIVE \r czu. 2

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Charles D. Boyley His Ahorney United States Patent PHOTOCONDUCTIVECONTROL Charles D. Bayley, Ipswich, Mass., assignor to General ElectricCompany, a corporation of New York ApplicationDecember 28, 1955,-SerialNo. 555,956

3 Claims. (Cl. 317-128) My invention relates to photoconductive controlcircuits particularly suited for turning street lights on and ofi atdesired ambient light levels.

Controllers using a photoemissive cell have heretofore been employed toturn on a street light at evening when daylight falls below apredeterminedlevel and to turn the street light off in the morning whendaylight rises to the point where artificial illumination :is no longerrequired. In these controllers the output'of the cell is amplified oneor more times and used to operate a direct current electromagnetic relaywhose contacts are connected in the lighting circuit. Since the increaseand decrease in current of the photoemissive cell is gradual inaccordance with its response to the gradual change in light level, aregenerative action'is built into the amplifier circuit so that aninitial gradual increase or decrease of current flow to the relaywinding is abruptly changed in response to this initial change incurrent flow in order to impart a snap action to the relay which willprevent destructive arcing at its contacts which would otherwise occurdueto the light contact pressure immediately preceding separation andclosure of its contacts.

Photoconductive cells have also been employed in light sensitivecontrols for turning street lights on and off at desired ambient lightlevels. These cells have not been connected in circuit with the windingof arelay across the alternating current source of supply because thegradual change in current through .the cell resulting from the gradualchange in light level to whichit responds produces contact chatter andarcing at the contacts of the relay which control the lamp current whichmay be of the order of amperes or more with an inrush of ten times thisvalue for tungsten filament lamps. This chattering is introduced by thecombined effects of frequency oscillations of the alternating current,slow rate of change in cell resistance and the slow rateof change of thesmall current in the operating winding of the relay. To overcome thesedifiiculties the alternating current of the supply has been rectifiedand fed through the photoconductor to the Winding of a sensitive directcurrent relay whose contacts are consequently not subjected to frequency oscillations and whose contacts are more positively closed inresponse to small changes in the control current supplied to itswinding. Usually, a condenser is connected across the winding of thisrelay to eliminate the etfect of pulsations in the rectified current. Inthese controls the contacts of the'direct current'relay are connected incircuit with the operatingswinding of a power relay across thealternating current supply and the contactsiof the power relaygareemploye'd' for controlling the lamp circuit current. Since the positiveaction of the direct current relay imparts a positive action to thecontacts of the power relay,- satisfactory operation is obtainable. Thedutyon the contacts ofthe direct current relay is light since only thesmall current of the winding of the power relay is controlled thereby.In thus applying photoconductive cells to street light- 2,913,53?Patented Nov. 1 7, 1 959 ing control, the use of a direct current pilotrelay to operate the. alternating current power or load circuit relay isin accordance with accepted practice for constructing other lightsensitive photoconductive controllers that have been used for dimmingautomobile headlights and for similar, control functions.

It is an object of my invention to provide a photoconductive controlcircuit which greatly reduces the number of control elements employedand provides for a positive action of the load circuit relay whosecontacts can consequently be located directly in the lamp circuitwithout being subjected to destructive arcing at the time they areopened and closed in response to the slow change in current flow throughthe photoconductor resulting from the slow change in light level towhich the photoconductor is subjected.

It is also an object of my invention to provide a control circuit whichis the ultimate in both simplicity and cost as compared to circuitsheretofore employed for eliminating undesirable contact chatter andarcing at relay contacts connected in the load circuit.

Further objects of my invention will become apparent from aconsideration of theembodiment thereof shown in the accompanying drawingrepresenting in schematic form a control circuit constructed inaccordance with these teachings.

In accordance with my invention, the operating winding of a snap-actionrelay is connected directly in series with a photoconductive cell acrossthe alternating current supply conductors and the comparatively heavycurrent of the lamp circuit is controlled by the abrupt opening andclosing of its contacts at predetermined current values in the operatingwinding of the relay. The snap action of the contacts of the relay isobtained by using any one of a number of over-center snapactionmechanisms for operating the switch contacts in response topredetermined changes in the energization of its winding. Such amechanism will maintain a positive pressure between the relay contactsuntil the mechanism passes through its dead center position so that atno time are the contacts of the relay subject to vibration and arcingdue to the frequency of the alternating cur- 7 rent supplied to itsoperating winding and the light pressure and slow movement of itscontacts when opening and closing.

In the diagrammatic representation of the embodiment of my inventionshown in the drawing, the operating winding 1 of a snap-action relay 2is connected directly in series circuit with a photoconductor 3 acrosssupply conductors 4 which are directly connectible with an alternatingcurrent source of supply of commercial voltage. The winding of the relayencircles its core structure 5 which, depending on the energization ofthe winding, attracts an armature 6 which is biased by a spring 7 to thefull line position shown in the drawing. The relay armature is connectedthrough an over-center snap-action mechanism comprising a bow spring 3to a flexible support member 9 for one of the contacts it? of a switchwhose other contact is mounted on a support member 11. When armature 6of the relay is in the full line position illustrated, the bow spring 8and contact support 9 are also in their full line positions illustrated;When the relay is energized the armature 6 moves to its dotted lineposition and the bow spring 8 passes through a dead center position tomove switch member9 to its dotted line position and open contac': 10 ofthe switch 9, 11. When the relay winding 1 is de-energized} the parts ofthe relay return to their full line positions again closing the contactsof switch 9, ll. The contacts 10 of this snap-actuated switch completethe connection of the lamp circuit 12 across the alternating currentsupply conductors 4. One or more lamps 3 13 are connected in multiple orseries circuit relation with one another for energization through thislamp circuit from the source of supply.

The particular over-center snap-action swltch mechanism represented inthe drawing is but one of many forms of such mechanism which maybeemployed. So long as the mechanism imparts a positive pressure to therelay contacts until over-center movement of the actuating mechanism isabout to begin and then rapidly opens or closes these contacts by aselfcompleting action so that the contacts pass through a position ofzero pressure substantially instantaneously, the requirements of mymvention will be satisfied. The mechanism may-be referred to as a toggleswitch without a zero contact pressure point or by the more commonlyaccepted definition of being a snap-action switch. In all instances, themechanism will be an over-center device with a rapid movement of theelements thereof through the dead center position of the device.

I prefer to employ a cadmium sulfide cell as the photoconductor althoughit is quite obvious that other types of photoconductive cells may beemployed. Since the relay that I have used requires about 6 milliamperesof exciting current to operate it, the cell, of course, must be capableof passing this current when excited by one foot candle of illuminationsince it is also desirable to have the controller operate with thissensitivity. I have used a microcrystalline layer type cadmium sulfidecell in which the sensitive material has been deposited by anevaporative process on its support member although it is, of course,possible to use any cadmium sulfide layer type cell such as those inwhich the sensitive layer has been applied to its support by spraying,silk screening, or simply spreading the material in a thin layer on itssupport. The cell that I have employed is of the interdigital electrodetype as has been indicated in the drawing. It, of course, does notnecessarily require this construction since other broad area type cellshaving area type electrodes, at least one of which is transparent, maybe used. In order to prevent destructive heating of the cell, inaccordance with my invention the impedance of the cell for thephotocurrent flow required to operate the relay controlled thereby ispreferably substantially equal to the impedance of the relay winding.This arrangement will impose a selflimiting condition on current flowthrough the cell whichiwill protect it from overheating. Both the celland the relay must be capable of withstanding the alternating currentline voltage to which they are directly subjected. The particular cellthat I have used, as previously'stated, is capable of supplying to arelay winding of about 10,000 ohms 6 milliamperes at a line voltage of120 volts and one foot candle of illumination when its impedance is ofthe order of 10,000 ohms. The dark impedance of this cell is of theorder of one megohm.

The arrangement I have provided is the ultimate in both simplicity andcost as compared with photoconductive controls heretofore proposed. Theuse of a snapaction relay having its winding connected directly inseries with the photoconductive cell across the alternating currentsupply conductors and having its contacts in the lighting circuit, makesit possible to eliminate the heretofore employed direct current circuitswhich required a rectifier and a capacitor for the operation of a directcurrent pilot relay which in turn controlled the energization of thewinding of a power relay w'hose contacts were in the lamp circuit. Thecontacts of a snap-action relay are under a positive pressure whenclosed and since these contacts are rapidly closed or opened atpredetermined current values, arcing at these contacts is reduced to aminimum and a controller embodying sucha relay will give the dependableand trouble-free operation required thereof. Obviously, my invention isnot limited .in its application to controlling street lights for whichit is particularly suited but may be used in other light sensitivecontrols where operating conditions are similar to those encountered instreet lighting control.

As previously indicated, various types of snap-action relays and varioustypes of photoconductive cells may be employed without departing fromthe spirit and scope of my invention. I consequently, intend to cover inthe appended claims all such modifications of my invention as fallwithin the true spirit and scope thereof.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is: v

1. A control circuit comprising supply conductors directly cqnnectiblewith an alternating current source of supply of commercial voltage, analternating current relay having contacts and means including a windingand an over-center snap-action mechanism operated thereby for movingsaid contacts substantially instantaneously into and out of a positivepressure engagement with one another when said mechanism passes throughits dead center position, a photoconductive cell operable at thecommercial Voltage of said supply conductors and capable of supplying anoperating value or" current to said winding of said relay, said cellhaving electrodes connected directly in series circuit with theoperating wind ing of said relay across said supply conductors, and loadcircuit conductors connected across said supply conductors through'thecontacts of said relay. v g V 2. A control circuit comprising supplyconductors directly connectible with an alternating supply of commercialvoltage, an alternating current relay having contacts and meansincluding a Winding and an over-center snap-action mechanism operatedthereby for moving said contacts substantially instantaneously into andout of a positive pressure engagement with one'ano-ther when saidmechanism passes through its dead center position, a photoconductivecell connected directly in series circuit with said operating winding ofsaid relay across saidsupply conductors, the impedance of said cell forthe photocurrent flow required to operate said relay being substantiallyequal to the impedance of said winding of said relay in order to limitcurrent flow in said circuit and the heating of said cell, and loadcircuit conductors connected across said supply conductors through thecontacts of said relay.

3. A control circuit comprising supply conductors, an alternatingcurrent relay having contacts and means including a winding and anover-center snap-actionmechanism operated thereby for moving saidcontacts substantially instantaneously into and out ofa positivepressure engagement with one another when said mechanism passes throughits dead center position, a photoconductive cell having a layer ofmicrocrystalline cadmium sulfide material suflicient in area to supplyan operating value of current to said winding of said relay andelectrodes in engagement with said layer of cadmium sulfide material,means for connecting said electrodes of said cell in series circuit withsaid Winding of said relay across said supply conductors, and loadcircuit conductors connected across said supply conductors through thecontacts of said relay.

References Cited in the file of this patent V UNITED STATES PATENTS Re.19,199

OTHER REFERENCES Relay Engineeringfi 1945, pub. by Struthers-Dunn, Inc.,Philadelphia, Pa. pp.'212-214 and 342-343,

